Add ERT for table alignment within Org buffer

* testing/lisp/test-org-table.el (test-org-table/align): New
ert-deftest for table alignment within Org buffer.
* testing/org-test.el (org-test-table-target-expect): New defun.
The new function org-test-table-target-expect is to simplify writing
spreadsheet ERT by providing just a target table to apply the formula
to, the expected result table and the table formula with optionally
additional variants. The variants typically are to check the same
result with a Lisp formula and a Calc formula. test-org-table.el is
also a howto example collection as a user documentation.